9.2.2. Steps of Construction

1. Plot the catchment boundary and mark all rain gauge stations with recorded rainfall values.

The first step in the Isohyetal Method involves defining the boundaries of the catchment area on a map. It is essential to accurately mark the locations of all rain gauge stations within this area, as they provide the rainfall data necessary for further analysis.

Think of this like mapping out a city (the catchment) and placing pins where all the weather stations (rain gauges) are located. Each station gives you temperature and rainfall readings, helping you understand the overall climate of the city.

2. Interpolate rainfall between nearby stations and draw isohyets (contour lines of equal rainfall).

In this step, after you've plotted the stations, you need to estimate the rainfall in areas between those stations. This is done through interpolation, which involves using the data from nearby stations to estimate what the rainfall might be in the spaces between them. Once these interpolated values are established, you can draw isohyets, which are lines that connect points with equal rainfall amounts.

Imagine you're drawing a map of a beach where the tide levels are different at various points. You would look at certain measuring points (like piers) and estimate the water level in the spaces between by comparing those measurements. The isohyets are like drawing lines to show where the tide is at the same level along the beach.

3. Divide the catchment into zones between isohyets.

Once the isohyets are drawn, the next step is to segment the catchment area into specific zones found between each pair of isohyets. Each zone represents a specific range of precipitation based on the rainfall values associated with the isohyets surrounding it.

Think of a cake that's divided into slices where each slice represents a different flavor. Each zone between isohyets is like a slice of cake, segmenting the catchment based on different rainfall areas.

4. Measure the area between each pair of isohyets.

In this step, you calculate the total area found between each pair of isohyets. This measurement is crucial for determining how much precipitation is contributed by each zone to the overall rainfall for the catchment.

Imagine cutting out different shapes from colored paper to create a collage. The area of each cut-out shape (the area between isohyets) determines how much material you have to make a certain part of the collage, just like how you calculate the area of each zone to understand rainfall distribution.

5. Assign the average precipitation to each zone, calculated as the mean of the two bounding isohyets.

After determining the area of each zone, it is time to assign an average precipitation value to that zone. This average is obtained by taking the mean of the precipitation values at the two isohyets that bound the zone. This ensures that each zone reflects a realistic contribution to the overall rainfall based on its position between the isohyets.

Imagine a temperature map where two cities near each other report different temperatures—one says 20°C and the other 30°C. If you're in between, you'd likely estimate your temperature to be around 25°C, representing a balanced average between the two. That's how the average precipitation is determined for each zone.

6. Compute the area-weighted average rainfall as: P_avg = Σ(A_i × P_i) / A_total, where A = area between two isohyets and P = average rainfall between two isohyets.

The final step involves calculating the overall average rainfall across the catchment using the area-weighted average rainfall formula, where you multiply the area of each zone by the average precipitation assigned to that zone, then sum it up and divide by the total catchment area. This accounts for the different contributions from each zone based on size and rainfall amount.

If you're budgeting for groceries, you'd consider both the price and the quantity of each item you want to buy. Here, the area (like quantity) and the rainfall amount (like price) are combined to give you a total average expenditure, ensuring that larger areas with more rainfall impact your total budget proportionally.

--